Strains of E. coli from human cases of diarrhea were screened for their ability to hemagglutinate guinea pig red blood cells in the presence of mannose at OC. These conditions detect colonization antigens, i.e., K88, K99, and the colonization factor of human strain H10407. The results of the HA assay are being compared to colonization of the small bowel in infant rabbits. Toxigenic, HA positive strains are currently undergoing genetic analysis of their plasmid composition. Use of curing agents has allowed us to isolate isogenic derivatives lacking one or more plasmids found in the parental strain. These isogenic sets are being compared in the suckling rabbit assay for colonization ability as well as for their ability to produce enterotoxins. We have examined the infant rabbit model of intestinal adhesion (colonization) and have isolated the specific HA receptor which we are presently chemically characterizing. This preparation is being used to produce antisera against the HA receptor. This monospecific sera will be used to screen other human toxigenic isolates for this antigen and to clarify the role of such pili in the specific attachment of toxigenic E. coli to the intestinal mucosa. BIBLIOGRAPHIC REFERENCE: Evans, D.G., Silver, R.P., Evans, D.J., Chase, D.G. and Gorbach, S.L. Plasmid-controlled colonization factor associated with virulence in Escherichia coli enterotoxic for humans. Infect. Immun. 12:656-667, 1975.